The biological method of-the sewage water treatment consists in utilizing the activated sludge formed by a mixture of various bacteria and small microorganisms. For its existence, this sludge needs organic substances contained in sewage water which is decomposited and thus cleared by these substances. The activating process is possible only due to a continuous oxidation introduced as a rule by blowing air into the activation tank.
For the sewage water treatment are used partly microorganisms seated firmly on their base in the form of various systems of biological filters and reactors wetted by sewage water, partly activating systems with suspended sludge where the sludge floccules are mixed together with sewage water and with air.
The sewage water treatment plants using the suspended sludge method know hirherto can be devided into systems using a continuous sewage water passage through the activation tank, and to systems using a discontinuous or intermittent passage.
In the continuous sewage water treatment systems, the sewage water is supplied after its coarse pre-treatment to the activation tank and, after a technologically necessitated period needed to its clearing, drained to a separated post-sedimentation tank together with the activated sludge. In this tank, the sludge is finally separated by sedimentation on the cleared water being drained away.
In the system using the discontinuous sewage water passage, the sewage water is supplied after its coarse pre-treatment to the activation tank either immediately or having been re-pumped from the equalizing tank. After the water treatment, the activation process is interrupted, i.e. the aeration and the water mixing, if any in the activation tank are stopped the cleared water being pumped or drawn by gravity to the drainage after the sludge sedimentation. After this, the activation tank is refilled the above described treatment cycle being repeated. Compared with the continuous sewage water treatment method, the post-sedimentation tanks can be omitted the activation one (SBR) being refilled in cycles.
The disadvantage of the above described sewage water activation treatment systems is their difficult utilization for small household treatment plants respecting in particular the demands connected with the treatment plant operation control.
In the activation treatment plants using the continuous sewage water passage, the sludge must be continuously re-pumped from the post-sedimentation tank to the sewage water inflow to the activation tank, As soon as the sludge concentration in this tank exceeds the permitted value, the excessive sludge must be pumped away from the sewage water treatment plant, A skilled operator is required to perform regular measuremants of the sludge concentration in the activation tank and to remove the sludge. In addition to this, a low sewage water inflow would cause an intermittent load of the activation tank. This would be followed by a deteriorated drainage water quality or by a necessity of overdimensioning the activation and the post-sedimentation tanks to obtain the required drainage water parameters.
With respect to a sludge accumulation in the activation tank, the hitherto known small sewage water treatment plants with a continuous water passage are designed either for a high sludge density where the sludge suspension must be maintained using a method with high demands on energy for up to 200 days till the sludge drainage without any interruptions of the water treatment plant function, or they require skilled operators draining regularly the sludge away from the activation tank. Both systems cannot be kept functioning for a longer time period without sewage water inflow due to the sludge autolysis followed by the sludge remowal from the activation process introduced stepwise due to the consumption of activation substances in the activation tank. In this way, the water treatment plant function is substantially affected. The activation sewage water treatment plants with discontinuous water passage (SBR) are earmarked by a relatively sophisticated control systems thus being excessively expensive for their use in low capacity sewage water sources.